Presently, glass plates are employed as members for openings in really large number of fields such as glass substrates for displays, particularly for glass substrates for flat panel displays such as liquid crystal displays, plasma displays, organic EL displays or field emission displays, for window glasses of architectures such as houses or buildings, or for window glasses of transportation vehicles such as automobiles, trains, airplanes or ships. These glass plates are formed from a molten glass by using a float method, a fusion method or a downdraw method. Further, the glass plates are obtained by applying a redraw process to glass plates formed by a primary forming.
Bubbles existing in these glass plates are problematic since they prevent visibility. For example, a thin glass plate of 3 mm thick or thinner is employed as a glass substrate for displays, and if a bubble larger than a predetermined size is existing in the thin glass plate, a white defect is formed in a screen of the display, which prevents visibility of the display. Further, a glass plate of 7 mm thick or thinner is employed as a photomask, and if a bubble of at least a predetermined size is existing in the glass plate, the bubble forms a defect of the photomask.
When a low material batch is processed in a melting process to produce a glass, gases such as CO2, H2O, O2 or SO2 are discharged, but a part of these gases remain in the molten glass as bubbles. When the molten glass is formed into a glass plate, the bubbles existing in the molten glass are each extended in a horizontal direction to have an elliptical shape. Such an elliptical bubble has larger maximum diameter than that of a spherical bubble before it is extended into an elliptical shape, and thus, the bubble particularly adversely affects the visibility of the glass plate.
Heretofore, reduction of the amount of bubbles existing in a glass plate, has been attempted by such methods as improvement of the structure of melting bath or a mixing mechanism inside the melting bath, selection of glass composition which prevents generation or growth of bubbles, or addition of small amount of additives for preventing generation or growth of bubbles. However, although it is possible to reduce the amount of bubbles existing in the glass plate by these methods, it has been difficult to make the amount of bubbles infinitely close to zero. Further, there are many objects to be studied in order to achieve improvement of equipment or change of glass composition, and such a study increasing production cost of glass plates. Further, there occurs e.g. an inconvenience that production of glass is prevented when these methods are carried out.
Patent Document 1 discloses a defect-repairing method for photomask glass that is a method of removing a fine defect existing in a photomask glass following by filling the position where the defect was present with a small glass piece and a liquid curable resin. However, in this method, since the position from which a fine defect is removed by e.g. a drill is filled with a small glass piece and a liquid curable resin, its operation is troublesome. For example, in order to fill the position from which a defect is removed by a drill, it is necessary to prepare a small glass piece of a desired size. Further, after the position is filled with a small glass piece and a liquid curable resin, a polishing operation of a photomask surface is required to achieve desired flatness. These problems become particularly significant in a case of repairing a defect existing in the central portion in the thickness direction of a glass thin plate. Further, it is difficult to make optical characteristics of a portion filled with the small glass piece and the liquid curable resin and those of other portions completely the same as each other, whereby there is a possibility that the portion filled with the small glass piece and the liquid curable resin becomes a new defect.    Patent Document 1: JP-A-10-239828